A multiple-input multiple-output (Multiple Input Multiple Output, MIMO) technology has been widely applied in a wireless communications system to improve a system capacity and ensure cell coverage. For example, multi-antenna based transmit diversity, open-loop/closed-loop spatial multiplexing, and demodulation reference signal (Demodulation Reference Signal, DM-RS) based multi-stream transmission are used in downlink in a Long Term Evolution (Long Term Evolution, LTE) system. The DM-RS based multi-stream transmission is a main transmission mode used in an LTE Advanced (LTE-A) system and a follow-up system.
In a conventional MIMO system, a codeword-to-pilot-port mapping relationship of data is fixed when a quantity of transmission layers, that is, a rank is fixed. In this process, multiple mapping relationships exist, and are mainly a codeword-to-layer mapping (Codeword-to-layer mapping) and a layer-to-pilot-port mapping. A pilot port includes a channel state information-reference signal (CSI-RS) port and a demodulation reference signal (DM-RS) port. A mapping relationship between a layer and a corresponding CSI-RS port may be further classified into a mapping relationship between the layer and a corresponding demodulation reference signal (DM-RS) port and a mapping relationship between the corresponding DM-RS port and the corresponding CSI-RS port. When the quantity of transmission layers is fixed, the fixed mapping relationship causes a waste of resources and low transmission efficiency.
Each antenna port for transmitting a reference signal is mapped to four antenna elements or four radio frequency units. The antenna port for transmitting a reference signal refers to a logical port used for transmission. Different weighted values are used to weight different antenna elements or different transceiver radio units (TxRU, transceiver radio unit) to obtain beams in different directions. The weighted values are complex numbers. Weighting performed on an antenna element or a TxRU corresponding to an antenna port for transmitting a reference signal may be a mapping relationship. FIG. 8a shows a case in which two antenna ports for transmitting a reference signal are mapped to four antenna elements. In the present invention, a port 0 (802) and a port 4 (803) are mapped to four antenna elements in the first column (801) in a first antenna array; a port 1 (805) and a port 5 (806) are mapped to four antenna elements in the second column (804) in the first antenna array; a port 2 (808) and a port 6 (809) are mapped to four antenna elements in the third column (807) in the first antenna array; and a port 3 (811) and a port 7 (812) are mapped to four antenna elements in the fourth column (810) in the first antenna array. The port 0 to the port 7 are antenna ports for transmitting a reference signal. The four antenna elements in the first column in the first antenna array are used as an example. Beams formed by using the four antenna elements in the first column in the first antenna array are formed by mapping the port 0 and the port 4 to the four antenna elements and by means of transmission. This specific mapping manner may be in a form of weighting. Weighted values that may be used for the port 0 are w0, w1, w2, and w3, and weighted values used for the port 4 that is mapped to the same four antenna elements (or TxRUs) are w4, w5, w6, and w7, where w0, w1, w2, w3, w4, w5, w6, and w7 are complex numbers. Therefore, a direction of a beam, formed by performing weighting by using w0, w1, w2, and w3, of the port 0 is a first direction (813), and a direction of a beam, formed by performing weighting by using w4, w5, w6, and w7, of the port 4 is a second direction (814). Specific beam orientations of the first direction and the second direction may be determined according to a scenario. For example, the first direction may point upwards relative to a horizontal plane, and the second direction may point downwards relative to the horizontal plane; or the first direction and the second direction may point to determined directions separately.
FIG. 8b is a schematic diagram of mapping antenna ports for transmitting a reference signal to two antenna elements. A port 4 (824) is mapped to two antenna elements 841 and 842 in the first column in a first antenna array. A Port 0 (823) is mapped to the other two antenna elements in the first column in the first antenna array. A mapping relationship of another column is similar. For the port 4, the antenna elements 841 and 842 corresponding to the port 0 are weighted, and so on.
The weighting described above may be precoding weighting, that is, when precoding is being performed on a signal by using a precoding matrix, weighting is performed by using the precoding matrix, which is equivalent to weighting performed on the signal. Alternatively, the weighting may be performed on a driver network of a radio frequency by using a circuit. Generally, if slashes shown in the figure indicate antenna elements, the weighting may be driver network weighting; if the slashes indicate TxRU radio units, the weighting manner may be precoding weighting. The precoding weighting may be baseband weighting.